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A new neurophysiological human thermal model based on thermoreceptor responses, the NHTM model, has been developed to predict regulatory responses and physiological variables in asymmetric transient environments. The passive system is based on Wissler's model, which is more complex and refined. Wissler's model segments the human body into 21 cylindrical parts. Each part is divided into 21 layers, 15 for the tissues and 6 for clothes, and each layer is divided into 12 angular sectors. Thus, we have 3780 nodes for the tissues and 1512 for clothes. The passive system simulates heat exchange within the body and between the body and the surroundings. The active system is composed of the thermoregulatory mechanisms, i.e., skin blood flow, shivering thermogenesis, and sweating. The skin blood flow model and the shivering model are based on thermoreceptor responses. The sweating model is that of Fiala et al. and is based on error signals. The NHTM model was compared with Wissler's model, and the results showed that a calculation based on neurophysiology can improve the performance of the thermoregulation model. The NHTM model was more accurate in the prediction of mean skin temperature, with a mean absolute error of 0.27 °C versus 0.80 °C for the original Wissler model. The prediction accuracy of the NHTM model for local skin temperatures and core temperature could be improved via an optimization method to prove the ability of the new thermoregulation model to fit with the physiological characteristics of different populations.

Purpose To compare two analytical methods for the estimation of the shivering onset inflection point, segmental regression and visual inspection of data, and to assess the test–retest reliability and validity of four metrics of shivering measurement; oxygen uptake (V̇O2), electromyography (EMG), mechanomyography (MMG) and bedside shivering assessment scale (BSAS). Methods Ten volunteers attended three identical experimental sessions involving passive deep-body cooling via cold water immersion at 10 °C. V̇O2, EMG, and MMG were continuously assessed, while the time elapsed at each BSAS stage was recorded. Metrics were graphed as a function of time and rectal temperature (Tre). Inflection points for intermittent and constant shivering were visually identified for every graph and compared to segmental regression. Results Excellent agreement was seen between segmental regression and visual inspection (ICC, 0.92). All measurement metrics presented good-to-excellent test–retest reliability (ICC’s > 0.75 and 0.90 respectively), with the exception of visual identification of intermittent shivering for V̇O2 measurement (ICC, 0.73) and segmental regression for EMG measurement (ICC, 0.74). In the assessment of signal-to-noise ratio (SNR), EMG showed the largest SNR at the point of shivering onset followed by MMG and finally V̇O2. Conclusions Segmental regression provides a successful analytical method for identifying shivering onset. Good-to-excellent reliability can be seen across V̇O2, EMG, MMG, and BSAS, yet given the observed lag times, SNRs, along with known advantages/disadvantaged of each metric, it is recommended that no single metric is used in isolation. An integrative, real-time measure of shivering is proposed.

Daily compensable cold exposure in humans reduces shivering by ~20% without changing total heat production, partly by increasing brown adipose tissue thermogenic capacity and activity. Although acclimation and acclimatization studies have long suggested that daily reductions in core temperature are essential to elicit significant metabolic changes in response to repeated cold exposure, this has never directly been demonstrated. The aim of the present study is to determine whether daily cold-water immersion, resulting in a significant fall in core temperature, can further reduce shivering intensity during mild acute cold exposure. Seven men underwent 1 h of daily cold-water immersion (14°C) for seven consecutive days. Immediately before and following the acclimation protocol, participants underwent a mild cold exposure using a novel skin temperature clamping cold exposure protocol to elicit the same thermogenic rate between trials. Metabolic heat production, shivering intensity, muscle recruitment pattern, and thermal sensation were measured throughout these experimental sessions. Uncompensable cold acclimation reduced total shivering intensity by 36% ( P = 0.003), without affecting whole body heat production, double what was previously shown from a 4-wk mild acclimation. This implies that nonshivering thermogenesis increased to supplement the reduction in the thermogenic contribution of shivering. As fuel selection did not change following the 7-day cold acclimation, we suggest that the nonshivering mechanism recruited must rely on a similar fuel mixture to produce this heat. The more significant reductions in shivering intensity compared with a longer mild cold acclimation suggest important differential metabolic responses, resulting from an uncompensable compared with compensable cold acclimation. NEW & NOTEWORTHY Several decades of research have been dedicated to reducing the presence of shivering during cold exposure. The present study aims to determine whether as little as seven consecutive days of cold-water immersion is sufficient to reduce shivering and increase nonshivering thermogenesis. We provide evidence that whole body nonshivering thermogenesis can be increased to offset a reduction in shivering activity to maintain endogenous heat production. This demonstrates that short, but intense cold stimulation can elicit rapid metabolic changes in humans, thereby improving our comfort and ability to perform various motor tasks in the cold. Further research is required to determine the nonshivering processes that are upregulated within this short time period.

Because the majority of cold exposure studies are constrained to short-term durations of several hours, the long-term metabolic demands of cold exposure, such as during survival situations, remain largely unknown. The present study provides the first estimates of thermogenic rate, oxidative fuel selection, and muscle recruitment during a 24-h cold-survival simulation. Using combined indirect calorimetry and electrophysiological and isotopic methods, changes in muscle glycogen, total carbohydrate, lipid, protein oxidation, muscle recruitment, and whole body thermogenic rate were determined in underfed and noncold-acclimatized men during a simulated accidental exposure to 7.5°C for 12 to 24 h. In noncold-acclimatized healthy men, cold exposure induced a decrease of ∼0.8°C in core temperature and a decrease of ∼6.1°C in mean skin temperature (range, 5.4-6.9°C). Results showed that total heat production increased by approximately 1.3- to 1.5-fold in the cold and remained constant throughout cold exposure. Interestingly, this constant rise in Ḣprod and shivering intensity was accompanied by a large modification in fuel selection that occurred between 6 and 12 h; total carbohydrate oxidation decreased by 2.4-fold, and lipid oxidation doubled progressively from baseline to 24 h. Clearly, such changes in fuel selection dramatically reduces the utilization of limited muscle glycogen reserves, thus extending the predicted time to muscle glycogen depletion to as much as 15 days rather than the previous estimates of approximately 30–40 h. Further research is needed to determine whether this would also be the case under different nutritional and/or colder conditions.

As endotherms, humans exposed to a compensable cold environment rely on an increase in thermogenic rate to counteract heat lost to the environment, thereby maintaining a stable core temperature. This review focuses primarily on the most important contributor of heat production in cold-exposed adult humans, shivering skeletal muscles. Specifically, it presents current understanding on (1) the origins of shivering, (2) the contribution of shivering to total heat production and (3) the metabolic requirements of shivering. Although shivering had commonly been measured as a metabolic outcome measure, considerable research is still needed to clearly identify the neuroanatomical structures and circuits that initiate and modulate shivering and drives the shivering patterns (continuous and burst shivering). One thing is clear, the thermogenic rate in humans can be maintained despite significant inter-individual differences in the thermogenic contribution of shivering, the muscles recruited in shivering, the burst shivering rate and the metabolic substrates used to support shivering. It has also become evident that the variability in burst shivering rate between individuals, despite not influencing heat production, does play a key role in orchestrating metabolic fuel selection in the cold. In addition, advances in our understanding of the thermogenic role of brown adipose tissue have been able to explain, at least in part, the large inter-individual differences in the contribution of shivering to total heat production. Whether these differences in the thermogenic role of shivering have any bearing on cold endurance and survival remains to be established. Despite the available research describing the relative thermogenic importance of shivering skeletal muscles in humans, the advancement in our understanding of how shivering is initiated and modulated is needed. Such research is critical to consider strategies to either reduce its role to improve occupational performance or exploit its metabolic potential for clinical purposes.

During cold exposure, increase in heat production is produced via the activation of shivering thermogenesis and nonshivering thermogenesis, the former being the main contributor to compensatory heat production in non-acclimatized humans. In rats, it has been demonstrated that shivering thermogenesis is modulated solely by skin thermoreceptors but this modulation has yet to be investigated in humans. The aim of this study was to determine if cold-induced shivering in humans can be modulated by cutaneous thermoreceptors in conditions where increases in heat loss can be adequately compensated by increases in thermogenic rate. Using a liquid-conditioned suit, six non-acclimatized men were exposed to cold (6 °C) for four 30 min periods, each of them separated by 15 min of heat exposure (33 °C). Core temperature remained stable throughout exposures whereas skin temperatures significantly decreased by 12% in average during the sequential cold/heat exposures compared to baseline ( p 2 /min, respectively) and were significantly reduced during 33 °C exposure (0.5±0.1% MVC, 0.25±0.0 L O 2 /min; p

The effect of iontophoretic administration of calcium ions to skin in the area of cold stimulus application on the thermal thresholds and the magnitude of cold defense responses in normotensive Wistar and hypertensive inherited stress-induced arterial hypertensive rats was studied. In thermoneutral conditions, administration of calcium ions was without effect on the measured thermoregulatory parameters. Under the effect of calcium, the thresholds of all the thermoregulatory responses to rapid cooling (such as heat loss, oxygen consumption, shivering) are lowered and the values of heat loss and shivering thermogenesis are considerably increased. All changes are more expressive in hypertensive rats. The increased sensitivity of hypertensives to calcium suggests that change in their calcium metabolism may be a cause of the observed shifts in the thermoregulatory response to cold.

A brief overview on thermogenic mechanisms in young precocial birds is given. While shivering thermogenesis is well documented in these birds, evidence for a facultative non-shivering component of heat production, comparable to that found in the brown adipose tissue of mammals, is ambiguous. One reason for this is the confusion between thermoregulatory and obligatory thermogenesis. In particular, the existence of a thermogenic reaction, even a futile one, does not by itself constitute proof of true thermoregulatory non-shivering thermogenesis. More probably, such a reaction is another obligatory component of heat production. Heat increment of feeding and motor activity are classical examples of such mechanisms. Thermogenesis arising from such mechanisms can often be adaptively used by the thermoregulatory systems in young birds, as well as in adults.

To understand the mechanisms of improvement of cold-induced heat production in aged mice following exercise training, the relative contributions of shivering and nonshivering thermogenesis to cold-induced metabolic responses were assessed in adult and aged C57BL/6J male mice, which inhabited sedentarily at room temperature, or were subjected either to a regimen of moderate intensity exercise training at 6°C, or to sedentary repeated exposures to the same temperature. The main findings were that (1) aged mice had greater cold-induced nonshivering thermogenesis, but lower shivering than adult mice; (2) exercise training in a cold environment enhanced cold-induced nonshivering thermogenesis in adult mice, but suppressed it in aged animals; (3) exercise training in a cold environment increased shivering thermogenesis in both age groups, but this increase was much greater in aged mice; (4) the increase of cold-induced shivering thermogenesis was mainly responsible for increased cold tolerance in aged mice after exercise training in a cold environment.

Goheen, M. S. L., M. B. Ducharme, G. P. Kenny, C. E. Johnston, John Frim, Gerald K. Bristow, and Gordon G. Giesbrecht.Efficacy of forced-air and inhalation rewarming by using a human model for severe hypothermia. J. Appl. Physiol. 83(5): 1635–1640, 1997.—We recently developed a nonshivering human model for severe hypothermia by using meperidine to inhibit shivering in mildly hypothermic subjects. This thermal model was used to evaluate warming techniques. On three occasions, eight subjects were immersed for ∼25 min in 9°C water. Meperidine (1.5 mg/kg) was injected before the subjects exited the water. Subjects were then removed, insulated, and rewarmed in an ambient temperature of −20°C with either 1) spontaneous rewarming (control), 2) inhalation rewarming with saturated air at ∼43°C, or 3) forced-air warming. Additional meperidine (to a maximum cumulative dose of 2.5 mg/kg) was given to maintain shivering inhibition. The core temperature afterdrop was 30–40% less during forced-air warming (0.9°C) than during control (1.4°C) and inhalation rewarming (1.2°C) ( P< 0.05). Rewarming rate was 6- to 10-fold greater during forced-air warming (2.40°C/h) than during control (0.41°C/h) and inhalation rewarming (0.23°C/h) ( P< 0.05). In nonshivering hypothermic subjects, forced-air warming provided a rewarming advantage, but inhalation rewarming did not.

Diabetes mellitus and food deprivation are two conditions known to significantly reduce the ability to generate body heat during periods of acute cold stress. The purpose of this study was to determine if shivering is attenuated in the urethane-anesthetized (1.5 g/kg; i.p.), streptozotocin-induced diabetic (STZ; n = 10) and food-deprived (12-hour nocturnal fast) rat (FD; n = 11) as colonic temperature (Tcol) declined from baseline (36 degrees C) to 28 degrees C. Shivering was assessed using the mean rectified electromyographic (EMG) signal obtained from indwelling bipolar electrodes placed in the gluteus superficialis muscle. Although the mean rectified EMG progressively increased (Por = 0.05) between Tcol of 33 degrees C to 28 degrees C and achieved peak activity (7.89 +/- 1.80 microV) at 29 degrees C in non-diabetic rats, shivering activity was virtually absent in the STZ group throughout cooling (e.g. peak EMG = 0.49 +/- 0.09 microV). The lack of shivering activity in STZ could partially explain the shorter time to reach 28 degrees C (STZ, 48.5 +/- 1.5 vs CON, 136.5 +/- 23.0 min; Por = 0.05) and the divergent trends in oxygen consumption (delta VO2) between STZ and non-diabetic rats. In the FD group, the mean peak rectified EMG activity (3.09 +/- 1.35 microV) was significantly lower (Por = 0.05) than the fed group. The peak delta VO2 from baseline (FD, +2.11 +/- 0.36 vs. CON, +4.51 +/- 0.50 mlO2/min) and the time taken to reach 28 degrees C (FD, 73.4 +/- 4.2 vs CON, 136.5 +/- 23.0 min) were statistically different (Por = 0.05) between groups. The results indicate that: 1) shivering thermogenesis is severely depressed and hypothermia accelerated in experimental diabetic animals as evidenced by the attenuation in mean rectified EMG and delta VO2, and 2) FD rats experienced a faster decline in colonic temperature than the fed group due, in part, to the relatively greater decline in shivering activity and oxygen consumption.

We measured daily energy expenditure (DEE) during the development periods of precocial chicks of five species of Arctic shorebirds spanning a broad range in size, in order to investigate the relationships between DEE, body size, and growth rate. We also quantified the effect of weather conditions on the energy expenditure of chicks to establish the impact of cold arctic weather on their time and energy budgets. We used the doubly labeled water method to measure DEE at ambient temperatures in an outside enclosure on the subarctic tundra at Churchill, Manitoba, Canada. Growth rate was highest in the smallest species, and in general decreased with adult size, as shown by the decrease in growth rate constant and later inflection point with increasing adult body mass. DEE ranged from 14 kJ day−1 in young chicks of the smaller species to 365 kJ day−1 in older chicks of the larger species. From hatching onward, DEE of shorebirds was high compared to DEE of chicks of altricial species, reflecting the high costs of locomotion and thermoregulation, and mass-specific DEE increased rapidly to adult levels. Chicks of smaller species metabolized more energy per unit mass and functioned at higher multiples of resting metabolic rate than chicks of larger species. As chicks of smaller species also had higher growth rates, and thus simultaneously invested in growth and mature function, it seems that shorebird chicks can adjust total metabolizable energy as well as its relative allocation between investment in growth and mature function. DEE was similar to cold-induced peak metabolic rates achieved by shivering thermogenesis. Because chicks were motionless during laboratory metabolism measurements, the relatively high level of DEE compared to these metabolic rates suggests that locomotion produces similar amounts of heat as shivering. Ambient temperature did not affect DEE in outdoor pens, although higher wind speed resulted in slightly increased energy expenditure. Heat produced by locomotion is possibly sufficient to sustain thermoregulation under local environments. Furthermore, when ambient temperatures are low, chicks spend more time brooding and less time foraging, which presumably counters the tendency to increase thermogenesis in response to cold.

During cold exposure, increase in heat production is produced via the activation of shivering thermogenesis and nonshivering thermogenesis, the former being the main contributor to compensatory hea...

The ontogeny of shivering thermogenesis was investigated in the altricial red-winged blackbird (Agelaius phoeniceus). Two indices of heat production – the rate of oxygen consumption of the bird and the electromyographic (EMG) activity of the pectoralis (PECT) and gastrocnemius (GAST) muscles – were measured simultaneously in adult and nestling red-winged blackbirds as they were subjected first to thermoneutral temperatures and subsequently to progressively colder ambient temperatures (Ta). The ontogenetic changes in both indices indicated that the capability for thermogenesis in nestling red-winged blackbirds improved markedly with age. The metabolic rates of 3-day-old nestlings decreased during exposure to gradually falling ambient temperatures; at best, these nestlings were only able to maintain mass-specific at levels similar to or slightly above the resting metabolic rate at thermoneutral temperatures (RMR) for a short time before metabolic rates decreased with further cooling. Shivering was detected only in the PECT muscles and was of a relatively low intensity (maximum of sevenfold increase in intensity over basal levels). The 5-day-old nestlings increased mass-specific modestly (approximately 1.4-fold) above RMR and attained slightly higher maximal factorial increases in the EMG activity of the PECT (maximum of 18-fold basal levels) when exposed to the same experimental conditions. Shivering was also detected in the GAST muscles of these birds. The most striking improvements in both measures observed during the nestling period occurred between day 5 and day 8. Eight-day-old nestlings increased metabolic rates by approximately 2-to 2.5-fold over basal levels and sustained these elevated rates for longer before becoming hypothermic. Both the PECT and GAST muscles contributed significantly to shivering thermogenesis, and these older nestlings attained much higher factorial increases in the intensity of shivering (up to 72-fold) during exposure to cold temperatures. In addition, both the range and magnitude of the dominant frequencies of muscle activity in the PECT increased during postnatal development. The PECT muscles were a principal site of shivering thermogenesis in all nestling and adult red-winged blackbirds studied here. Shivering in these muscles was a ‘first line defense’ against cold; the threshold temperature for shivering in the PECT muscles coincided with the lower critical temperature for oxygen consumption (TLC), and the subsequent increases in EMG activity in this muscle with further cooling correlated well with the corresponding increases in mass-specific .

Body-to-body contact is often recommended for rewarming mildly hypothermic victims in the field. This procedure involves a euthermic individual donating heat to the recipient by direct contact in an insulated bag. However, this technique has not been critically evaluated and may not be beneficial because there is limited direct contact between recipient and donor, peripheral vasoconstriction may impair heat transfer to the core, skin warming may blunt the recipient's shivering response, and cold stress to the donor may be excessive. The present study was designed to evaluate whether donation of heat by a donor would be sufficient to enhance rewarming of a hypothermic subject (recipient). Six pairs of recipients (5 men, 1 woman) and donors (2 men, 4 women) participated in the study. Esophageal and skin temperatures, cutaneous heat flux, and oxygen consumption were measured. Recipients were immersed in 8 degrees C water until esophageal temperature decreased to a mean of 34.6 +/- 0.7 degrees C (SD). They then were rewarmed by one of three methods: rewarming by the endogenous heat generated by shivering only (SH), body-to-body rewarming (BB), or rewarming with a constant-heat source manikin (MAN). Mean afterdrop for the three conditions was 0.54 +/- 0.2, 0.54 +/- 0.2, and 0.57 +/- 0.2 degrees C for SH, BB, and MAN, respectively (NS), and the rate of rewarming was 2.40 +/- 0.8, 2.46 +/- 1.1 and 2.55 +/- 0.9 degrees C/h for SH, BB, and MAN, respectively (NS).(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of acute hypercapnia on human thermoregulation during cold exposure were investigated by immersion of eight male subjects to the neck in a 15 °C water bath until their core temperatures dropped to 35 °C or until 1 h of immersion had elapsed. Air was inspired throughout each experiment, with the exception of a 15-min period commencing with the attainment of an esophageal temperature (Tes) of 36.5 °C, during which subjects inspired a gas mixture containing 4% CO2, 20% O2, and 76% N2. Oxygen uptake ([Formula: see text], L∙min−1), inspired minute ventilation ([Formula: see text], L∙min−1), esophageal temperature (Tes, °C), rectal temperature (Tre, °C), mean unweighted skin temperature (Tsk, °C), mean heat flux (Q, W∙m−2), and electromyographic activity (EMG, mV) of the trapezius and masseter muscles were recorded continuously. [Formula: see text] and integrated EMG activity (IEMG) were used as the primary indicators of shivering thermogenesis. Shivering EMG was attenuated immediately following the switch of the inhaled gas mixture from air to 4% CO2. For both the massetter and trapezius muscles the IEMG was significantly suppressed (p < 0.05) during the hypercapnic period. The IEMG values preceding the switch to the hypercapnic mixture were 15% greater than those during the CO2 period. Similarly, IEMG values in the post-CO2 period were 55% greater than during the CO2 period. It is concluded that acute periods of hypercapnia during cold exposure may result in transient suppression of shivering tremor, but this does not appear to affect thermal balance, as reflected in the absence of any significant effect on Tes.Key words: temperature regulation, carbon dioxide, hypothermia, diving, cooling, cold water immersion, carbon dioxide retention.

The present study investigated the effect of non-uniform skin temperature distribution on thermoregulatory responses and subjective thermal sensation during water immersion. Ten healthy male subjects carried out 60 min water immersion twice, once with uniform (UST) and once with non-uniform (NUST) skin temperature. In UST condition, subjects immersed at 29 degrees C in naked condition, while in NUST condition, subjects immersed at 26 degrees C with partial coverage wetsuit (PCWS). The PCWS covers trunk region, upper arms, and thighs. The non-uniform skin temperature distribution, higher at trunk and lower at distal extremities, was observed in NUST condition. Shivering thermogenesis was not influenced by the skin temperature distribution at the experimental condition of this study. On the other hand, the tissue insulation (I (tissue)) was significantly higher in NUST condition compared to the UST condition. The increment of I (tissue) might have been caused by the peripheral vasoconstriction induced by the cold input from the distal extremities in NUST condition. The higher I (tissue) in NUST condition might lead to the significantly higher esophageal temperature compared to UST condition. No difference was observed in thermal sensation between the two conditions. Subjects felt slightly more comfortable in NUST condition than in UST condition. In conclusion, the non-uniform skin temperature distribution, higher at trunk and lower at distal extremities, might affect the peripheral vasoconstriction to increase the I (tissue). On the other hand, shivering thermogenesis and subjective thermal sensation were not affected by the non-uniform skin temperature distribution at the present experimental condition.

In humans, the relative importance of oxidative fuels for sustaining shivering during passive hypothermic recovery or rewarming is still unclear. The main goals of this study were 1) to quantify the respective contributions of lipids and carbohydrates (CHO) during passive rewarming and 2) to determine the effects of precooling exercise on the pattern of fuel utilization. With indirect calorimetry methodologies, changes in fuel metabolism were quantified in nonacclimatized adult men shivering to rewarm from moderate hypothermia (core temperature ∼34.5°C) not following (Con) or following a precooling exercise at 75% V̇o2max for 15 min (Pre-CE). As hypothermic individuals shiver to normothermia, results showed that CHO dominate at all shivering intensities above 50% Shivpeak, while lipids were preferred at lower intensities. This change in the relative importance of CHO and lipids to total heat production was dictated entirely by modulating CHO oxidation rate, which decreased by as much as 10-fold from the beginning to the end of rewarming (from 1,611 ± 396 to 141 ± 361 mg/min for Con and 1,555 ± 230 to 207 ± 261 mg/min for Pre-CE). In contrast, lipid oxidation rate remained constant and low (relatively to maximal rates at exercise) throughout rewarming, averaging 183 ± 141 for Con and 207 ± 118 mg lipids/min for Pre-CE. In addition, this pattern of fuel selection remained the same between treatments. We concluded that fuel selection is regulated entirely by changes in CHO oxidation rate. Further research should focus on establishing the exact regulatory processes involved in achieving this large upregulation of CHO utilization rate following hypothermia.

Experiments were carried out to determine the role of nitric oxide in mediating autonomic and behavioral thermoregulatory control in rat pups on postnatal days 1-2, 5-6, and 10-11. For an experiment, each pup received a subcutaneous injection of vehicle, NG-nitro-d-arginine methyl ester (d-NAME; 100 mg/kg), or NG-nitro-l-arginine methyl ester (l-NAME; 100 mg/kg) before being placed in a metabolic chamber or in a thermocline with a linear temperature gradient of 23 to 43°C. In the metabolic chamber, oxygen consumption and core temperature were measured as ambient temperature was decreased from 40 to 15°C over a 60-min period. Decreasing ambient temperature elicited an increase in oxygen consumption in all age groups that received vehicle or d-NAME. The lower critical temperature and peak oxygen consumption upon exposure to cold after vehicle were 41 ± 10 ml · kg-1 · min-1 at 30°C, 43 ± 12 ml · kg-1 · min-1 at 28°C, and 55 ± 11 ml · kg-1 · min-1 at 25°C in the 1- to 2-, 5- to 6-, and 10- to 11-day-old pups, respectively. Administration of l-NAME abolished the oxygen consumption response to cold in the 1- to 2- and 5- to 6-day-old pups and significantly attenuated the oxygen consumption response to cold in the 10- to 11-day-old pups. Selected ambient temperature in the thermocline was not significantly affected by prior administration of d-NAME or l-NAME compared with vehicle. Thus our data provide evidence that the nitric oxide system plays a role in mediating autonomic but not behavioral thermoregulatory control in rat pups during early postnatal maturation.

Turkey (Meleagris gallopavo) and guinea fowl (Numida meleagris) chicks (0-27 days posthatch) were exposed to decreasing or increasing ambient temperatures. Root mean square electromyographic activity of musculus pectoralis (m. pect.) and musculus iliotibialis (m. iliot.) was recorded simultaneously with O2 consumption and CO2 production. From both muscles, relative mass, water fraction and fibre type were determined. M. iliot. participated in shivering from hatching onwards. The relationship between its root mean square electromyographic activity and ambient temperature resembled that of metabolic rate and ambient temperature, and the shivering threshold temperature was indistinguishable from the lower critical temperature. This suggests that the leg muscles are major contributors to shivering thermogenesis. M. pect. participated in shivering only at days 6-20 in turkeys and at days 6-10 in guinea fowl. Both water fraction and histological analysis indicated that m. pect. was less developed than m. iliot. at hatching. We hypothesize that a minimal level of maturity is required before a muscle can participate in shivering, which is probably represented by a water fraction of about 0.85. Both species recruited the aerobic leg muscles first; the anaerobic breast muscle was recruited only when the rate of mass-specific heat loss was high.

The present study investigated whether nitrous oxide (N2O) attenuates shivering thermogenesis during cold water immersion in a dose-dependent manner. Seven male subjects were immersed to the neck for 60 min in 20 degrees C water on five separate occasions while breathing either air (AIR) or a normoxic mixture of 10, 15, 20, or 25% N2O balanced with N2. All N2O concentrations investigated caused a significant (P < 0.02) reduction in shivering thermogenesis compared with AIR. Despite similar heat flux from the skin, the relative changes in esophageal temperature from resting preimmersion levels were significantly greater (P < 0.05) during the N2O trials compared with AIR, with no significant difference among the N2O conditions. A dose-dependent trend in the perception of thermal comfort was observed for the N2O conditions. It is concluded that shivering thermogenesis, and thus thermal balance, is affected to the same degree for the range of inspired N2O concentrations investigated, with no discernable dose-dependent effect.

Thermoregulatory responses of eight healthy subjects (six men and two women) were compared when they were head-out immersed in 15 degrees C water at both 1 and 6 ATA. Both trials were conducted in a hyperbaric chamber. During the immersions, esophageal temperature (T(es)) and skin temperature at two sites (chest and calf) were recorded at minute intervals. Oxygen uptake was determined at 5-min intervals with the Douglas bag method. The order of the two trials was alternated. The rate of T(es) cooling was greater during the 6-ATA trial [2.1 +/- 0.5 degrees C/h (SE)] than during the 1-ATA trial (1.3 +/- 0.5 degrees C/h; P < 0.01). Despite the greater rate of core cooling, and presumably a greater thermal drive for shivering, the oxygen uptake response for a similar decrement in T(es) was lower during exposure to 6 than to 1 ATA (P < 0.05). Also, for similar displacement in T(es), the subjects perceived the immersions at 6 ATA to be less cold than those at 1 ATA (P < 0.05). It is concluded that the development of hypothermia in compressed-air divers may be due, in large part, to the attenuation of heat production and cold perception. Most likely, the observed effects on the autonomic responses and thermal perception are due to an inhibitory action of hyperbaric nitrogen on central neural structures involved in temperature regulation.

(I) Shivering intensity and metabolic rate were determined in Large White pigs aged 2, 24, 48 h and 5 d, at temperatures ranging from thermoneutrality (36°C) to cold (20°C). (2) Shivering is the main heat producing mechanism, the absence of nonshivering thermogenesis being implied by both the absence of delay between the onset of shivering (Stt) and the increase in metabolic rate (Lct) and by the linearity of the relationship between metabolic rate and shivering intensity in the cold. (3) For a comparable thermal demand, shivering intensity decreased with age whereas cold induced heat production remained constant, which suggests that the thermogenic efficiency of shivering is improved during the first 5 days of life.

1. To test whether the preoptic area controls only non-shivering and the spinal cord only shivering thermogenesis, ten rats were chronically implanted with a preoptic and a spinal cord thermode each. The following were then studied: (a) the effect of propranolol (8 mg/kg.hr) on the metabolic response to cooling the preoptic area, and the spinal cord, (b) the effect of exogenous noradrenaline (0.5 mg/kg) on the metabolic response to cooling the preoptic area, and the spinal cord, and (c) the effect of warming the preoptic area on the metabolic response to cooling the spinal cord, and vice versa. 2. Administration of propranolol inhibited the metabolic response to cooling each of the thermosensitive areas, but the response to cooling the preoptic area was more strongly inhibited than that to cooling the spinal cord. 3. Administration of exogenous noradrenaline did not prevent the metabolic response to cooling either the preoptic area or the spinal cord. 4. Warming the spinal cord completely inhibited the metabolic response to cooling the preoptic area, and warming the preoptic area fully inhibited the metabolic response to cooling the spinal cord. 5. It is concluded that exogenous noradrenaline underestimates the capacity for non-shivering thermogenesis, and that both thermosensitive areas can control both forms of thermogenesis, but that the preoptic area threshold of non-shivering thermogenesis is probably lower than that of shivering, while the spinal cord threshold of shivering is probably lower than that of non-shivering thermogenesis.

Restraint hypothermia has often been described, but its cause has never been clarified. We hypothesized that it might be due to a suppression of shivering thermogenesis. Thus, we restrained conscious rats in an ambient temperature of 2 degrees C while measuring rectal (Tre) and tail skin temperatures, metabolic rate (MR), and shivering activity. When rats were cold exposed but not restrained, Tre fell 1.4 +/- 0.2 degrees C (SE) during the 1st h. When these same rats were restrained, Tre fell at a rate of 6.5 +/- 0.2 degrees C/h. MR averaged 15.7 +/- 1.4 W/kg for the unrestrained rats, but it averaged only 9.0 +/- 1.1 W/kg for the restrained rats. The restrained rats showed no signs of shivering. The animals were then subjected to a restraint adaptation regimen and then reexposed to cold. Restraint now produced a fall in Tre of only 2.6 +/- 0.7 degrees C/h. The animals shivered and generated an MR of 15.8 +/- 0.9 W/kg. Naive rats became hypothermic because restraint suppressed shivering activity. However, adapted rats continued to shiver and remained normothermic. We suggest that a stressful or threatening situation, such as restraint for a naive rat, inhibits shivering and leads to hypothermia in a cold environment. This would not occur in adapted rats because restraint is no longer stressful.

Physiological thermogenesis in intemal organs as well as main muscle groups is one of the most important subjects in the study of human adaptability to cold environment. The experiments of the prediction of obligatory non-shivering thermogenesis were performed under optimal thermal condition in summer and in winter by using a prediction method of local muscle energy metabolic rate and quasi-basal metabolic rate B1 (Yokayama, 1980). B1 means the sum of metabolic rates other than those of main muscle groups. The subjects were eleven Japanese males in sumrner and ten Japanese males in winter. Subject YS was longitudinally examined to confirm seasonal change of obligatory non-shivering thermogenesis. Subject YS was performed to examine regulatory non-shivering thermogenesis and shivering thermogene-sis by using nine resting and static exercising items of back cold and front cold under three cold environments (T=-l0°C, -5°C and 0°C. In the summer experiments Bls were 32.43 (mean) ±12.59 (S.D.) kcal/h and B1/A= 19.81 ±9.26 kcal/m2 h. On the other hand, the results of the winter experiments were 67.32±7.83 kcal/h and B1/A=38.90±5.27 kcal/m3h. This tendency of the seasonal variation that the quasi-basal metabolic rate B1 in winter was greater than in summer was coincident with the results in winter and in summer obtained from the same subject YS. In subject YS the predicted B1S were 29.87 in summer, 57.84 in December and 55.55 and 57.92 kcal/h in March. In order to evaluate regulatory non-shivering thermogenesis, the predictions of B1 were performed from item-e of back cold and front cold under cold environments. B1 under T=-5°C and T=0°C were 78.19 and 83.13 kcal/h respectively. Therefore the value of regulatory non-shivering thermogenesis was about 22 kcal/h under the present experimental conditions. The shivering thermogenesis was predicted for six to nine muscle groups in nine resting and exercising items of back cold and front cold under three cold environments

The purpose of the present study was to clarify the importance of skeletal muscle glycogen as a fuel for shivering thermogenesis in humans during cold-water immersion. Fourteen seminude subjects were immersed to the shoulders in 18 degrees C water for 90 min or until rectal temperature (Tre) decreased to 35.5 degrees C. Biopsies from the vastus lateralis muscle and venous blood samples were obtained before and immediately after the immersion. Metabolic rate increased during the immersion to 3.5 +/- 0.3 (SE) times resting values, whereas Tre decreased by 0.9 degrees C to approximately 35.8 degrees C at the end of the immersion. Intramuscular glycogen concentration in the vastus lateralis decreased from 410 +/- 15 to 332 +/- 18 mmol glucose/kg dry muscle, with each subject showing a decrease (P less than 0.001). Plasma volume decreased (P less than 0.001) markedly during the immersion (-24 +/- 1%). After correcting for this decrease, blood lactate and plasma glycerol levels increased by 60 (P less than 0.05) and 38% (P less than 0.01), respectively, whereas plasma glucose levels were reduced by 20% after the immersion (P less than 0.001). The mean expiratory exchange ratio showed a biphasic pattern, increasing initially during the first 30 min of the immersion from 0.80 +/- 0.06 to 0.85 +/- 0.05 (P less than 0.01) and decreasing thereafter toward basal values. The results demonstrate clearly that intramuscular glycogen reserves are used as a metabolic substrate to fuel intensive thermogenic shivering activity of human skeletal muscle.

Cardiorespiratory responses of the conscious pigeon to (1) spinal cord heating to 43.7 C in a cold environment ($T_{a} = 5 C$) and (2) spinal cord cooling to 37.5 C in a hot environment ($T_{a} = 39-40 C$) were measured. Ambient cooling caused increases in respiration and cardiac output that were proportional to the increased heat production caused by shivering thermogenesis. Under this condition, selective heating of the spinal cord suppressed shivering, whereupon heat production and cardiorespiratory responses returned rapidly to control levels. Selective cooling of the spinal cord suppressed panting caused by ambient heating. Cardiovascular and blood gas changes accompanying panting were small; any responses that did occur returned rapidly to control levels during spinal cord cooling. The results on the conscious pigeon show that selective thermal stimulation of the spinal cord, when applied in the opposite direction to an external temperature stress, can inhibit not only shivering or panting but also ...

Existing stimulus-response expressions predicting shivering thermogenesis are based on static peripheral, core, and central temperatures. These expressions predict a linear increase in metabolic heat production in response to displacements of either central or core and peripheral temperatures from set-point temperature values. A model of shivering thermogenesis has been developed incorporating the nonlinear static and dynamic characteristics of thermosensitive neural structures in the body. The parameters defining receptor activities, dynamics of response, regional summation, and thermoneutral set points are based on data published in the literature. Predictions of metabolic heat generation (in units of ml O2 ·kg-1 ·min-1) are derived by integrating excitatory and inhibitory thermogenic drives, resulting from thermoreceptor stimulation in various regions of the body. The relative gains of the thermogenic drives were determined by least-squares regression using empirical data from a series of cold water immersions. The present model demonstrated improved simulations of shivering thermogenesis during cooling and rewarming, when compared to existing stimulus-response predictive expressions.

1. Shivering in response to changes in ambient or core temperature has been measured in control rats and rats injured by bilateral hind-limb ischaemia, both with and without fur. 2. Removal of the fur increased the threshold ambient temperature for the onset of shivering. 3. This threshold was lower than in uninjured controls during the latter half of a 4 hr period of bilateral hind-limb ischaemia in rats with or without fur. 4. The threshold was also lower than in uninjured controls after the period of bilateral hind-limb ischaemia but was not significantly different from that during the period of ischaemia. 5. The alterations in threshold were not accompanied by any significant changes in the slope of the regression line relating intensity of shivering and ambient temperature. 6. When the core temperature fell after limb ischaemia to levels which provoked shivering in uninjured controls, no shivering was seen in the injured rats. 7. These results are interpreted as further evidence for the lowering of the ‘set-point’ for thermogenesis after injury in the rat and are discussed in relation to the changes in body temperature after injury in that species.

An evaluation was made of the effects of an acute exercise bout on nonshivering thermogenesis (NST) in cold-acclimated rats (4 degrees C for 6 weeks) and shivering thermogenesis in 24 degrees C-acclimated rats (24 degrees C for 6 weeks). Assessment techniques included indirect calorimetry during treadmill running and brown adipose tissue (BAT) mitochondrial guanosine diphosphate (GDP) binding immediately following a treadmill run. Calorimetric results for 24 degrees C-acclimated rats running at 4 degrees C indicated total substitution of shivering thermogenesis by exercise-derived heat. No difference in GDP-binding, an index of BAT nonshivering thermogenic activity, was observed between exercised and nonexercised 24 degrees C-acclimated rats. Calorimetric results for cold-acclimated rats running at 4 degrees C indicated a total suppression in the energy cost associated with NST, exercise-derived heat replacing or substituting for NST. Examining BAT properties in the exercised cold-acclimated rats revealed a significant 40% decrease in BAT mitochondrial GDP-binding. These results suggest that during running, metabolic heat due to the exercise totally replaces shivering in 24 degrees C-acclimated rats and totally replaces BAT nonshivering thermogenesis in cold-acclimated rats.

Publisher Summary This chapter describes the importance of the pilomotor response in temperature regulation. The capability of a perfect regulation of body temperature is only found in mammalian and avian species. The members of the two classes of vertebrates are equipped with the ability to regulate autonomically their heat dissipation and their internal heat production. Some autonomic features of heat loss and heat conservation are already established in the ectothermic reptiles. These animals are unable to increase their internal heat production by shivering thermogenesis. The capability of shivering in homeotherms appears to be enclosed with the development of the insulation of the body surface. This assumption is supported by observations of the thermogenesis during the ontogeny of endothermic animals. In newborn mammals and birds, shivering follows when the growth of hairs or feathers results in a good peripheral insulation. In contrast to mammals, the thickness of plumage of the birds does not vary so much between summer and winter and between inhabitants of warm and cold regions with the exception of antarctic birds.

The influence of ambient temperature and lighting on the duration of tonic immobility (TI) in birds was studied in conjunction with simultaneous shivering measurements. Contrary to what has been found in mammals, low ambient temperature did not disrupt TI in pigeons and house sparrows: the TI durations were equal at +2°C and +25°C. Electromyographic recordings in pigeons showed that after an initial suppression, shivering soon reappeared and reached normal levels during TI at +2°C. When the duration of TI was prolonged (indicating slower recovery of postural activity) by experimental manipulation (darkness), shivering likewise reappeared more slowly. The ability of birds to maintain shivering during TI, and the congruent changes in the duration of TI and the intensity of shivering suggest that shivering has neuromuscular and evolutionary connections to the postural activity of skeletal muscles also in birds. The difference between birds and mammals in reference to the effects of low ambient temperature on TI may be related to a differential recruitment of muscles for shivering thermogenesis.

No change could be detected in the adenyl cylase system (basal activity, noradrenaline-stimulated activity, adrenaline-stimulated activity, fluoride-stimulated activity) of skeletal muscle of cold-acclimated rats. It is concluded that the enhancement of metabolic response to noradrenaline during cold-acclimation, which occurs principally in skeletal muscle, can not be attributed to an alteration in this component of the receptor system for noradrenaline.An increase in the activity of the catecholamine-stimulated and fluoride-stimulated adenyl cyclase of skeletal muscle occurs during the 1st week of exposure to cold and has disappeared by the time the rats are fully acclimated. This increase coincides with the period of shivering thermogenesis rather than with the development of nonshivering thermogenesis; it may be related to the intense and repeated stimulation of plasma membrane associated with shivering.

1. 1. Electromyographic, thermal and metabolic measurements were made on pigeons subjected to stepwise ambient cooling. Subsequent signal analysis of the EMG-recordings from the pectoral muscle showed that the mean rectified value (Umrv) is the most reliable EMG-based indicator of metabolic heat production (M). 2. 2. The muscle-abdomen temperature difference was linearly related to M and correlated strongly with Umrv. 3. 3. The long-term and short-term fluctuations of shivering intensity (Umrv) highly synchronous in the two main muscles, m. pectoralis and m. supracoracoideus. 4. 4. The EMG-signal approached a Gaussian process at high shivering intensities. Evaluation of the changes in EMG median frequency suggested a size-dependent recruitment of motor units for shivering. 5. 5. It is pointed out that Umrv and the integrated value of EMG are interconvertible and that Umrv should be preferred, because it has more physiological relevance.

Serum free fatty acid versus glycerol ratios were decreased in human newborn infants during pyrexia or cold injury, but remained practically unchanged in older infants and children. Decreased ratios in the newborn resulted exclusively from an increase in serum glycerol. In children a parallel increase in both serum glycerol and free fatty acids was observed in pyrexia. The results suggest that in pyrexia both newborn infants and older children behave as if they were cold, mobilizing the mechanisms of nonshivering and shivering thermogenesis respectively to adjust the temperature of their body at a relatively higher reference level.

Rectal temperature cooling rate (Tre) is a consequence of an imbalance between heat production and heat loss mechanisms. However, Tre is often related to body insulation, as determined by the thickness of the subcutaneous fat layer, without consideration of the contribution of shivering thermogenesis. The present study investigated the combined effects of insulation and heat production on Tre during cold water (10°and 20°C immersion. It is concluded that relating core temperature cooling rate simply to the insulative adipose layer is inadequate. On the basis of the present results, it is suggested that the following variables be taken into account when attempting to model cooling rate of immersed individuals : 1) The rate of heat production. 2) The insulation provided by the adipose layer. In addition, other tissues may contribute to the overall body insulation. During prolonged immersion total body insulation may vary and will also be affected by the level of peripheral perfusion. 3) The rate of storage of body heat will depend on body composition. It may be insufficient to assume identical specific heat of the body for a range of body types, when determining total body insulation.

We inadvertently subjected a group of goats to 5 mo of cold exposure (mean minimum temperature less than −13°C) during an experiment designed to examine the effects of training by daily running on one member of each sibling pair. During the three coldest months, the sedentary but cold-exposed goats experienced a 34% increase in maximal oxygen uptake (V˙o 2 max, P < 0.01) and a 29% increase in running speed at maximal ( P < 0.05). When temperatures increased in the spring, both oxygen uptake and running speed decreased. We interpret these findings as evidence that cold is a sufficient stimulus to invoke the development of aerobic structures in muscle and that these structures subsequently can be utilized for the novel task of running. When the experiment was subsequently repeated without the cold exposure, running speed andV˙o 2 max of trained animals increased less than in either group of cold-exposed animals. However, the cost of transport of these warm runners was lower than either group of cold-exposed animals (from 13–19%, P < 0.0001). Thus, although aerobic capacity was increased with acclimation to severe winter weather, cold-acclimated goats operated with lower efficiency during locomotion.